In electrophotography, an image comprising a pattern of electrostatic potential, also referred to as an electrostatic latent image, is formed on a surface of an electrophotographic element comprising at least two layers: a photoconductive layer and an electrically conductive substrate. The electrostatic latent image can be formed by a variety of mechanisms, for example, by image-wise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
Among the many different kinds of photoconductive materials that have been employed in electrophotographic elements are phthalocyanine pigments such as titanyl phthalocyanine and titanyl tetrafluorophthalocyanines. Electrophotographic recording elements, containing such pigments as charge-generation materials, are useful in electrophotographic laser beam printers because of their capability for providing good photosensitivity in the near infrared region of the electromagnetic spectrum, that is, in the range of 700–900 nm.
In a photoconductive layer produced from a liquid coating composition that includes the titanyl phthalocyanine pigment and a solvent solution of polymeric binder, it is necessary that the titanyl phthalocyanine pigment be in a highly photoconductive form, either crystalline or amorphous, and in a sufficiently stable dispersion to permit its application as a very thin layer having high electrophotographic speed in the near infrared region.
A variety of methods have been used to produce suitable forms of titanyl phthalocyanine having differing crystallographic characteristics. U.S. Pat. No. 5,166,339 includes a table of polymorphs of unsubstituted titanyl phthalocyanine in which materials bearing multiple designations are grouped as four types. Many phthalocyanine pigments are discussed in P. M. Borsenberger and D. S. Weiss, Organic Photoreceptors for Imaging Systems, Marcel Dekker, Inc., New York, pp. 338–391.
In one type of preparation, commonly referred to as “acid-pasting”, crude titanyl phthalocyanine is dissolved in an acid solution, which is then diluted with a non-solvent to precipitate the titanyl phthalocyanine product. In another type of procedure, the crude titanyl phthalocyanine is milled, generally with particular milling media. Additionally, some preparations include a combination of techniques or modify a previously prepared titanyl phthalocyanine.
U.S. Pat. Nos. 5,238,764 and 5,238,766, the disclosures of which are incorporated herein by reference, teach that titanyl fluorophthalocyanine products of acid-pasting and salt-milling procedures, unlike unsubstituted titanyl phthalocyanine, suffer a significant reduction in near infrared sensitivity when they are dispersed in a solvent such as methanol or tetrahydrofuran, which has a gammac hydrogen bonding parameter value greater than 9.0. These patents further teach that this reduction in sensitivity can be prevented by first contacting the titanyl fluorophthalocyanine with a material having a gammac hydrogen bonding parameter of less than 8.0.
U.S. Pat. No. 5,629,418, the disclosure of which is incorporated herein by reference, describes a method for preparing titanyl fluorophthalocyanine that provides the steps of: dissolving titanyl fluorophthalocyanine in acid to form a solution; admixing the solution and water to precipitate out amorphous titanyl fluorophthalocyanine; washing the amorphous titanyl fluorophthalocyanine until substantially all of the acid is removed and contacting it with an organic solvent, which results in the conversion of the amorphous material to high crystallinity titanyl fluorophthalocyanine, the amorphous titanyl fluorophthalocyanine having been maintained in contact with water continuously from its precipitation to its conversion to a crystalline form.
U.S. Pat. No. 5,523,189, the disclosure of which is incorporated herein by reference, describes an electrophotographic element including a charge generation layer that includes a binder in which is dispersed a physical mixture of a high speed titanyl fluorophthalocyanine having a first intensity peak with respect to X-rays characteristic of Cu Kα at a wavelength of 1.541 Å of the Bragg angle 2θ at 27°±0.2°, and a second intensity peak at 7.3°±0.2°, the second peak having an intensity relative to the first peak of less than 60 percent; and a low speed titanyl fluorophthalocyanine having a first intensity peak with respect to X-rays characteristic of Cu Kα at a wavelength of 1.541 Å of the Bragg angle 2θ at 6.7°±0.2°, and a second intensity peak at 23°±0.2°, the second peak having an intensity relative to the first peak of less than 50 percent.
U.S. Pat. No. 5,773,181, the disclosure of which is incorporated herein by reference, describes a method for preparing a phthalocyanine composition providing the steps of: synthesizing a crystalline product comprising a mixture of five different unsubstituted or fluorosubstituted phthalocyanines, wherein a central M moiety bonded to the four inner nitrogen atoms of the phthalocyanine nuclei represents a pair of hydrogen atoms or a covalent or coordinate bonded moiety, including an atom selected from the group consisting of: Li, Na, K, Be, Mg, Ca, Ba, Sc, Y, La, Ac, Ti, Zr, Hf, V, Nb, Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Al, Ga, In, Tl, Si, Ge, Sn, Pb, As, and Sb, with M preferably representing Ti═O;
increasing the amorphous character of the mixture of phthalocyanines as determined by X-ray crystallography using X-radiation characteristic of Cu Kα at a wavelength of 1.541 Å of the Bragg angle 2θ to provide an amorphous pigment mixture;
contacting the amorphous pigment mixture with organic solvent having a gammac hydrogen bonding parameter of less than 8.0; and
prior to the contacting, substantially excluding the amorphous pigment mixture from contact with organic solvent having a gammac hydrogen bonding parameter greater than 9.0.
The particle size distribution and stability of charge generation dispersions are very important for providing uniform charge generation layer in order to control generation of “breakdown spots” and minimize the granularity of prints. U.S. Pat. Nos. 5,614,342 and 5,766,810, the disclosures of which are incorporated herein by reference, describes a method for preparing cocrystals of titanyl fluorophthalocyanine and unsubstituted titanyl phthalocyanine that provides the steps of: admixing crude titanyl phthalocyanine and crude titanyl fluorophthalocyanine to provide an amorphous pigment mixture, as determined by X-ray crystallography using X-radiation characteristic of Cu Kα at a wavelength of 1.541 Å of the Bragg angle 2θ; contacting the amorphous pigment mixture with an organic solvent having a gammac hydrogen bonding parameter of less than 8:0; and, prior to contacting, substantially excluding the amorphous pigment mixture from contact with an organic solvent having a gammac hydrogen bonding parameter greater than 9.0. The amorphization step must be substantially complete so as to break the large primary particles of the starting crude pigments and thereby lower the average particle size of the final cocrystalline mixture. Substantially complete amorphization of the crude pigments is also necessary to prevent degradation of the dark decay characteristics of the final cocrystal; small amounts of crude pigments having inherently high dark decay that are not amorphized would not be affected by the subsequent solvent treatment and therefore would retain their high dark decay characteristics, causing degradation of the dark decay property of the final cocrystalline product.
The final step in the method of U.S. Pat. No. 5,614,342 entails contacting the cocrystalline pigment with an organic solvent having a gammac hydrogen bonding parameter of less than 8:0, for example, a chlorinated solvent such as dichloromethane or 1,1,2 trichloroethane. After the organic solvent treatment, the pigment has to be filtered and dried, a procedure that is rather time consuming.
In the previously mentioned U.S. patent application Ser. No. 10/655,388, a method of isolating an amorphous mixture of titanyl phthalocyanine and fluorinated titanyl phthalocyanine is disclosed that involves the use of water as an isolation medium. It has now unexpectedly been found that the amorphous mixture made by this method can undergo heat-induced crystallization in the dry state to form a cocrystalline composition different from that obtained in the presence of organic solvents or water.